Vice President Biden ostensibly took the stage Thursday night to square off against the man who seeks to replace him, Rep. Paul Ryan (R-Wis.).

But almost from the moment the debate began in Danville, Ky., it was clear that Biden’s real mission was to do what President Obama failed to in his own lackluster performance against Republican nominee Mitt Romney last week.

And in doing so, Biden was seeking to repair the damage from that stumble on Obama’s part, which has moved poll numbers across the map in the direction of the GOP ticket.

On issue after issue, Biden defended what the Obama administration has done and ­painted the Republican ticket as out of step with the concerns of average Americans.

“A bunch of malarkey,” said Biden when Ryan warned that cutting defense spending would make the country weak.

“I’ve never met two guys who are more down on America across the board,” added the vice president when the subject turned to the economy.

Ryan, on the other hand, chose to play it cautiously, seeking to avoid mistakes, to display the mastery of fiscal policy that he has gained as chairman of the House Budget Committee and to reassure swing voters that the policies of a Romney presidency would not decimate social programs.

It will be several days before it becomes clear whether either candidate did much to change the dynamic of the race; the instant polls suggested that viewers considered the matchup pretty much of a draw.

When he was talking, Biden dominated the debate and an opponent 27 years his junior.

And though Biden is a man with a reputation for making gaffes, his worst moments came when he wasn’t talking but remained under the unblinking gaze of the camera.

As Ryan spoke, the split screen picked up Biden’s grins and chortles, suggesting a dismissiveness and scorn for the views of an opponent he repeatedly called “my friend,” and he appeared to make no attempt to suppress them.

Although Biden’s frequent interruptions probably helped revive the spirits of the Democratic faithful, they may have been too much for less partisan viewers.

“Mr. Vice President, I know you’re under a lot of duress to make up for lost ground, but I think people would be better served if we don’t keep interrupting each other,” Ryan said at one point.

Biden made many of the arguments that Obama, for whatever reason, failed to in his debate against Romney.

For instance, Biden raised Romney’s controversial comment, caught on videotape, in which the Republican presidential nominee suggested that the 47 percent of Americans who do not pay income taxes are government-dependent freeloaders who consider themselves victims.

“These people are my mom and dad, the people I grew up with, my neighbors,” Biden said. “They pay more effective tax [through payroll and other ­taxes] than Governor Romney pays in his federal income tax. They are elderly people who, in fact, are living off of Social Security. They are veterans and people fighting in Afghanistan right now.”

Ryan defended Romney, drawing laughs from the audience when he noted: “The vice president very well knows that sometimes the words don’t come out of your mouth the right way.”

Biden retorted: “But I always say what I mean. And so does Romney.”

One of Ryan’s missions was to show a softer side of Romney, a son of privilege whom many view as out of touch with average Americans.

Ryan told a story of how Romney had come to the aid of a family whose children were hurt in an auto accident, and he praised the GOP presidential nominee for giving 30 percent of his income to charity.

“Stop talking about how you care about people,” Biden retorted. “Show me something. Show me a policy.”

Biden came to the debate stage with far more experience than Ryan in that kind of setting.

Biden was elected to the Senate when Ryan was only 2, has run for president twice and was matched up against then-Alaska Gov. Sarah Palin in a vice-presidential debate four years ago.

Adding to the challenge for Ryan was the fact that before he joined Romney’s ticket, he had established a reputation as an intellectual force within his party, as the author of a fiscal plan that has been embraced by Republicans in the House.

So, in addition to promoting Romney’s policies, he had to defend his own — including a controversial proposal to transform Medicare into a system under which the elderly would have the option of shopping for coverage with government vouchers.

The undercard match of this presidential season proved to be a spirited encounter, producing far more sparks than the top of the ticket had in their first debate.

But history suggests that vice-presidential debates rarely do much to change the course of a campaign. And any effect this one may have will almost certainly be superseded when Obama and Romney meet Tuesday for their second debate, at Hofstra University on Long Island. The format will be a town hall, in which audience members get to ask most of the questions.

And instead of relying on his vice president as a proxy, Obama will get his own shot at a do-over.

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